Conventionally, when a signal having a wide dynamic range such as UMTS (Universal Mobile Telecommunications System) is amplitude-modulated, there is a proposal of a scheme of reducing a necessary dynamic range by amplitude-limiting an amplitude component thereof in advance (e.g., see Patent Document 1).
FIG. 1 shows an example of a modulation circuit apparatus that performs such amplitude limitation. Modulation circuit apparatus 101 shown in FIG. 1 is configured with angle modulator 102, voltage control circuit 103, amplitude modulator 104 and amplitude limitation section 109.
Angle modulator 102 is a circuit that angle-modulates a carrier wave using a phase signal inputted from second input terminal 106.
Voltage control circuit 103 is a circuit used to supply stable power to amplitude modulator 104 for amplifying an amplitude signal inputted from first input terminal 105.
Amplitude modulator 104 is a circuit that amplitude-modulates a signal inputted from angle modulator 102 using a signal inputted from voltage control circuit 103.
Amplitude limitation section 109 is a circuit that forms, if the level of an upper waveform of an amplitude signal inputted from first input terminal 105 exceeds a first predetermined value, the upper waveform of the amplitude signal so that the level of the amplitude signal of the exceeding portion is truncated to the first predetermined value and/or forms, if the level of the upper waveform of the inputted amplitude signal is greater than the first predetermined value and the level of a lower waveform of the amplitude signal falls below a second predetermined value, the lower waveform of the amplitude signal so that the level of the amplitude signal of the lower portion is truncated to the second predetermined value.
Next, the operation of modulation circuit apparatus 101 in FIG. 1 will be explained.
A data generator (not shown) generates an amplitude signal and a phase signal from an inputted signal. The amplitude signal generated by the data generator is inputted to first input terminal 105 and the phase signal generated by the data generator is inputted to second input terminal 106. A supply voltage is supplied to voltage control circuit 103 from power terminal 107.
Amplitude limitation section 109 limits the amplitude component of the amplitude signal inputted to first input terminal 105 according to the first predetermined value and/or second predetermined value and outputs the amplitude-limited signal to voltage control circuit 103. Voltage control circuit 103 amplifies the signal inputted from amplitude limitation section 109 and then outputs the amplified signal to amplitude modulator 104. By using this voltage control circuit 103, it is possible to supply a stable voltage to amplitude modulator 104. That is, it is possible to avoid voltage changes due to changes in input impedance of amplitude modulator 104 or the like.
Furthermore, the phase signal inputted to second input terminal 106 is inputted to angle modulator 102. Angle modulator 102 angle-modulates the inputted phase signal using a carrier wave and outputs the angle-modulated phase signal to amplitude modulator 104. This angle-modulated signal becomes an angle modulated wave of a constant envelope as shown in the figure. Amplitude modulator 104 then amplitude-modulates the inputted angle-modulated signal using the voltage supplied from voltage control circuit 103 and outputs the amplitude-modulated signal from output terminal 108. Patent Document 1: Japanese Patent Application Laid-Open No. 2005-45782